PTH both builds and breaks down bone, and the pathways by which PTH mediates these actions are beginning to be identified. Prior research suggests that RANKL, a molecule important in bone metabolism, responds to PTH, and that if the RANKL is inactivated, PTH is shifted towards building bone. The investigators will study the effect of Denosumab, a therapeutic agent that binds to and inactivates RANKL, in 28 postmenopausal women with PHPT. Our hypothesis is that Denosumab will increase bone mineral density in primary hyperparathyroidism.